Field of the Invention
The present invention relates to a speaker system in which an acoustic diaphragm and an image projection screen are incorporated as a unit.
Current progress of technology in the field of audio-visual systems has given rise to a demand for a system which would enable one to enjoy images projected on a large screen with powerful sound In general, speaker systems used in combination with large-size image projection screen are sorted into three types: a first type in which speakers are disposed behind the screen, a second type in which speakers are disposed on both lateral sides or below the image projection screen and a third type in which the image projection screen itself vibrates so as to function as an acoustic diaphragm. The system of the first type is used, for example, in cinema houses. In the system of this type, the acoustic characteristic is impaired by the screen which is disposed on the front side of the speaker. In addition, the brightness of the image on the screen is inevitably low due to porosity of the screen which is as high as 10 to 20%. The system of the second type has a wide use including home audio-visual systems This system is disadvantageous in that a large space is required for placing large speakers which can reproduce powerful sound, in addition to the space for installation of the screen. In particular, a problem has been encountered with this system in that the unbalance between the realism of the image and the sound level is increased as the size of the image increases. Namely, the sound level cannot be increased in such a manner as to match for enhanced realism of the image projected on a large screen.
A description will be given hereunder as to the third type of system which is most relevant to the present invention, with specific reference to FIGS. 1 and 2a and 2b.
Referring to FIG. 1, a known speaker system of the type in which an image projection screen itself vibrates has a diaphragm or curved screen 1 and diaphragm driving devices 2 which are secured to the lower end of the diaphragm 1. The driving devices 2 drive the diaphragm in the directions of the double-head arrow (b) perpendicular to the plane of the diaphragm by making use of the curvature of the diaphragm 1 so as to generate sound waves.
FIGS. 2a and 2b show, in plan and in section, another known speaker system of the type which vibrates a screen serving also as a diaphragm. In this system, a screen 1 serving as a diaphragm, driven by driving sources 2, has endless grooves 3 formed in the diaphragm 1. The stiffness of the diaphragm 1 is much smaller in the region where the endless grooves 3 are formed than in other regions. The vibration, therefore, takes place in the regions 4 surrounded by the endless grooves and is not transmitted to other regions. That is, only the regions 4 actually serve as diaphragms.
In the arrangement shown in FIG. 1, however, the diaphragm is limited only to the curved screen. This system, therefore, has only a limited application. In addition, sound waves transmitted from the diaphragm forwardly and backwardly interfere with each other by diffraction around the diaphragm in such a manner that sound waves of inverse phases cancel each other. In consequence, the level of the sound pressure is significantly reduced particularly in a bass or low-pitched sound region.
The speaker system shown in FIGS. 2a and 2b encounters a problem in that the stress is concentrated to the regions of the endless grooves 3 where the thickness of the diaphragm 1 is smaller than in other regions, so that the diaphragm 1 tends to be broken when the diaphragm 1 vibrates. In addition, in order to obtain a large sound pressure level with the vibration regions having limited areas, it is necessary that these vibration regions vibrate with a large amplitude. Such a large vibration undesirably causes unfavorable effects on the image, e.g., oscillation of the image. The interference between the sound waves of opposite phases also is encountered with this type of speaker system.